Zincate zinc plating bath

ABSTRACT

To provide a zincate zinc plating bath capable of efficiently forming a zinc plating film with high brightness and high qualities even at a low current density. The zincate zinc plating bath is used in electroplating for depositing a metal containing at least zinc and forming a zinc plating film. It contains a primary brightener having a function of reducing the size crystals of the metal to be deposited and a plating accelerator containing a heterocyclic compound having a predetermined structure such as 2,5-dimercapto-1,3,4-thiadiazole.

TECHNICAL FIELD

The present invention relates to a zincate zinc plating bath used forforming a zinc plating film on automotive steel plates, bolts, nuts, andthe like by using electroplating.

BACKGROUND ART

Zinc plating films have been used widely for various ordinary thingsincluding automotive steel sheets and mechanical parts formed of a steelmaterial such as bolts and nuts for the improvement of corrosionresistance. Zinc alloy plating films made of, for example, zinc-nickelalloy, zinc-iron alloy, or tin-zinc alloy have also been used widely tosatisfy demands for the improvement of heat resistance or salt waterresistance.

Zinc plating films are formed using electroplating which is performed bysupplying an electric current while dipping a material to be plated in aplating bath, and thereby electrifying the material to be plated.Plating baths for forming a zinc plating film can be roughly classifiedinto an alkaline bath and an acid bath. Examples of the alkaline bathinclude a cyanide bath and a zincate zinc plating bath, while those ofthe acid bath include a zinc chloride bath and a zinc sulfate bath. Fromthese plating baths, a proper one is selected as needed, depending onthe desired conditions such as hardness and brightness of a zinc platingfilm, shape and size of a material to be plated, and workingenvironment.

Among them, the zincate zinc plating bath has been preferred andemployed industrially because it does not use a cyanide which willbecome a burden on effluent treatment, a simple bath compositionfacilitates control of it, and this plating bath can be used even forsmall stamping parts, bolts and nuts. Zinc plating films formed using azincate zinc plating bath are however inferior in uniformelectrodeposition and covering power compared with films formed using acyanide bath which is a similar alkaline bath. Even though, the zincatezinc plating bath is very effective for forming a zinc plating filmunder an industrial environment and many improved baths have beendeveloped conventionally (refer to, for example, Patent Document 1).

Patent Document 1 discloses a zincate zinc plating bath containing abath-soluble quaternary ammonium polymer. This zincate zinc plating bathcan be mentioned as one of the excellent baths developed in recentyears. With this zincate zinc plating bath, a zinc plating filmexhibiting uniform electrodeposition and having high brightness can beformed on long materials or materials having a complex shape whichtherefore differ in the magnitude of current density.

Prior Art Document Patent Document

Patent Document 1: JP-A-8-209379

In the zincate zinc plating bath disclosed in Patent Document 1,however, uniform electrodeposition is improved by suppressing a platingefficiency at a high current portion when electroplating which causes awide range of current density distribution is performed and therebydecreasing a difference in thickness of the zinc plating film betweenthe high current portion and a low current portion. With this zincatezinc plating bath, a thin zinc plating film having a uniform thicknesscan therefore be formed. There is accordingly a demand for an effectthat cannot be found from the above-described conventionally knownzincate zinc plating bath. Described specifically, in electroplatingcausing a wide-range of current density distribution, there is a demandfor a zincate zinc plating bath capable of forming a zinc plating filmwhich is entirely thick, exhibits high uniform electrodeposition, andhas high brightness mainly by improving the plating efficiency at a lowcurrent density. When the plating efficiency is enhanced, theconventionally known zincate zinc plating bath has problems such asdeterioration in uniform electrodeposition and throwing power.

With the above problems in view, an object of the present invention isto provide a zincate zinc plating bath capable of efficiently forming azinc plating film even when electroplating is performed at a low currentdensity. Described specifically, the object is to provide a zincate zincplating bath having a high plating efficiency and high throwing powerwhen electroplating is performed at a low current density. Anotherobject is to provide, when electroplating which causes a wide range ofcurrent density distribution including a low current density (from 0.05to 1.0 A/dm²) is performed, a zincate zinc plating bath capable offorming a thick zinc plating film and exhibiting highly uniformelectrodeposition mainly by improving the plating efficiency at a lowcurrent density. A further object of the present invention is to providea zincate zinc plating bath capable of forming a zinc plating film withhigh brightness and high qualities.

SUMMARY OF THE INVENTION

With a view to satisfying the above objects, the present inventors havecarried out an extensive investigation of a variety of compounds thatcan be contained in a primary brightener, a plating accelerator, and asecondary brightener and completed the present invention. In short, thepresent invention provides a zincate zinc plating bath which will bedescribed below.

[1] A zincate zinc plating bath for depositing a metal containing atleast zinc to form a zinc plating film by using electroplating,containing a primary brightener having a function of reducing the sizeof crystals of the metal thus deposited and a plating acceleratorcontaining a heterocyclic compound represented by the following chemicalformula (I):

(in the chemical formula (I), n stands for a natural number from 1 to 3,R¹ and R² may be same or different and each represents hydrogen,CH₂COOH, CH₂CH₂OH, or CH₂CH(OH)CH₂Cl), and/or a derivative of theheterocyclic compound.

[2] The zincate zinc plating bath as described above in [1], wherein theheterocyclic compound and/or the derivative of the heterocyclic compoundis contained in an amount of 0.01 g/L or greater but not greater than1.0 g/L.

[3] The zincate zinc plating bath as described above in [1] or [2],wherein a water soluble cationic macromolecular compound is contained asthe primary brightener.

[4] The zincate zinc plating bath as described above in [3], wherein acompound represented by the following chemical formula (II):

(in the chemical formula (II), n stands for a natural number of 1 orgreater, R¹, R², R³, and R⁴ may be the same or different and eachrepresents hydrogen, methyl, ethyl, isopropyl, 2-hydroxylethyl-CH₂CH₂(OCH₂CH₂) OH (X standing for a natural number from 0 to 6), or2-hydroxyethyl-CH₂CH₂ (OCCH₂CH₂)_(x)OH (X standing for a natural numberfrom 0 to 6), R⁵ represents (CH₂)₂—O—(CH₂)₂, (CH₂)₂—O—(CH₂)—O—(CH₂)₂, orCH₂—CHOH—CH₂—O—CH₂—CHOH—CH₂, and Y represents S or O) is contained asthe water soluble cationic macromolecular compound.

[5] The zincate zinc plating bath as described above in any of [1] to[4], further containing a secondary brightener having a function ofplanarizing the zinc plating film.

[6] The zincate zinc plating bath as described above in [5], wherein atleast one compound selected from the group consisting of organicaldehydes and heterocyclic compounds is contained as the secondarybrightener.

[7] The zincate zinc plating bath as described above in [6], wherein atleast either one of an aromatic aldehyde or a pyridinium compound iscontained as the secondary brightener.

[8] The zincate zinc plating bath as described above as described abovein any of [1] to [7], containing zinc ions in an amount of 5 g/L orgreater but not greater than 20 g/L.

[9] The zincate zinc plating bath as described above as described abovein any of [1] to [8], containing sodium hydroxide in an amount of 80 g/Lor greater but not greater than 250 g/L.

[10] The zincate zinc plating bath as described above in any of [1] to[9], containing at least either one of nickel ions or iron ions.

[11] The zincate zinc plating bath as described above in [10], whereinthe nickel ions are contained in an amount of 100 mg/L or greater butnot greater than 4000 mg/L.

[12] The zincate zinc plating bath as described above in [10] or [11],wherein the iron ions are contained in an amount of 10 mg/L or greaterbut not greater than 150 mg/L.

The zincate zinc plating bath of the present invention enables efficientformation of a zinc plating film. Described specifically, it exhibits ahigh plating efficiency and a high throwing power when electroplating isperformed at a low current density. In addition, when electroplating isperformed at a wide range of current densities including a low currentdensity (from 0.05 to 1.0 A/dm²), the zincate zinc plating bath of thepresent invention exhibits highly uniform electrodeposition and enablesthe formation of a thick zinc plating film mainly by improving theplating efficiency at a low current density. Further, the zincate zincplating bath of the present invention enables the formation of a zincplating film with high brightness and high qualities when it contains awater soluble cationic macromolecular compound (represented by thebelow-described chemical formula (III)) having a specific structure asthe primary brightener and contains also a secondary brightener.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] FIG. 1 is a schematic view showing adsorption of a heterocycliccompound contained as a plating accelerator to a surface to be plated.

[FIG. 2] FIG. 2 illustrates the relationship between current density andplating efficiency in the zincate zinc plating bath of the presentinvention and a conventionally known zincate zinc plating bath.

[FIG. 3] FIG. 3 illustrates the relationship between current density andthickness of the zinc plating film in the zincate zinc plating bath(Example 9) of the present invention.

MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will next be described. The presentinvention is not limited to or by the following embodiments. Changes,modifications or improvements can be made without departing from thescope of the present invention.

1. Zincate Zinc Plating Bath:

1-1. Outline of the Zincate Zinc Plating Bath of the Present Invention:

The zincate zinc plating bath of the present invention (which willhereinafter be called “plating bath of the present invention”) is fordepositing a metal containing at least zinc to form a zinc plating filmby using electroplating. The plating bath of the present invention is azincate zinc plating bath containing a primary brightener having afunction of reducing the size of crystals of a metal to be deposited, aplating accelerator containing a heterocyclic compound represented bythe below chemical formula (I) and/or a derivative of the heterocycliccompound. The plating bath of the present invention can further containa secondary brightener having a function of planarizing the zinc platingfilm.

(In the chemical formula (I), n stands for a natural number from 1 to 3,R¹ and R² may be same or different and each represents hydrogen,CH₂COOH, CH₂CH₂OH, or CH₂CH(OH)CH₂Cl).

The plating bath of the present invention contains, similar to theconventionally known zincate zinc plating bath, zincate zinc([Zn(OH)₄]²⁻). The term “primary brightener” as used herein is similarto an agent which is called “primary brightener”, carrier, stressinhibitor, or the like by those skilled in the plating field. Theprimary brightener has a function of reducing the size of crystals of ametal thus deposited to form a smooth zinc plating film and also has afunction of relaxing an internal stress of the zinc plating film. Theterm “secondary brightener” as used herein is similar to an agent called“secondary brightener”, “leveler”, or the like by those skilled in theplating field. The secondary brightener has a planarizing effect on azinc plating film when the zinc plating film is formed byelectroplating.

These primary and secondary brighteners have, as these terms suggest, afunction of forming a zinc plating film with brightness. In particular,using the primary brightener and the secondary brightener in combinationcan accelerate reduction of the size of a deposited metal and give amirror surface brightness to the zinc plating film. In the plating bathof the present invention, a compound contained in the primary brightenerand the secondary brightener can be selected, depending on theappearance of the zinc plating film such as matte plating,half-brightness plating, brightness plating (which will be describedlater in detail).

The term “plating accelerator” as used herein means an additive having adeposition accelerating action which is added for plating by thoseskilled in the plating field. The plating accelerator is a substanceaccelerating an electrode reaction of plating and increasing a currentdensity. Incidentally, in this specification, a plating film containingzinc as a main metal component is collectively called “zinc platingfilm” and when a plating film containing an alloy of zinc and anothermetal as the component thereof should be called in distinction from thezinc plating film, a description will be made, while calling it “zincalloy plating film” (for example, Zn—Ni alloy plating film).

The plating bath of the present invention enables the efficientformation of a zinc plating film even when electroplating is performedin a low current region and further enables the formation of a zincplating film with high brightness and high qualities under predeterminedconditions. The plating bath of the present invention exhibits highlyuniform electrodeposition, high throwing power, and high platingefficiency even when electroplating is performed in a low currentregion. The plating bath of the present invention can therefore be usedeffectively for barrel plating in which electroplating is usuallyperformed in a low current region (which will be described later indetail).

Components of the plating bath of the present invention will next bedescribed. First, zinc and alkali used as “essential components” will bedescribed and then, “plating accelerator”, “primary brightener”, and“secondary brightener” will be described in detail successively.

1-2. Essential Components of the Plating Bath of the Present Invention

The plating bath of the present invention contains, similar to theconventionally known zincate zinc plating bath, zincate zinc([Zn(OH)₄]²⁻). The zincate zinc can be prepared by using, as needed, themethod ordinarily employed by those skilled in the plating technology.For example, to obtain a plating bath having zincate zinc dissolvedtherein, it is recommended to prepare zincate zinc by dissolving zincoxide (ZnO) in an aqueous alkali solution such as an aqueous sodiumhydroxide (NaOH) solution similar to the conventionally known method.

The plating bath of the present invention contains zinc ions preferablyin an amount of 5 g/L or greater but not greater than 20 g/L from thestandpoint of practical use of a zinc plating film relating to thequalities thereof (appearance, film thickness, ease of after treatment,corrosion resistance). Amounts of zinc ions contained in the platingbath less than 5 g/L are not preferred because they deteriorateappearance of the zinc plating film and deteriorate corrosion resistancedue to a decrease in the thickness of the zinc plating film. Amounts ofzinc ions contained in the plating bath exceeding 20 g/L, on the otherhand, are not preferred because they cause poor appearance of the zincplating film or deterioration in a throwing power. As the amount of zincions contained in the plating bath, an amount of zinc ions (Zn²⁺)determined by conversion of the mass of raw materials such as zinc oxide(ZnO) in terms of a molecular weight can be used.

The plating bath of the present invention contains sodium hydroxide(NaOH) preferably in an amount of 80 g/L or greater but not greater than250 g/L from the standpoint of stability of the plating bath andpractical use of a zinc plating film relating to the qualities thereof(appearance, film thickness, ease of after treatment, corrosionresistance). Amounts of sodium hydroxide contained in the plating bathless than 80 g/L are not preferred because the plating bath loses itsstability and causes poor appearance of the zinc plating film ordeterioration in throwing power. Amounts of sodium hydroxide containedin the plating bath exceeding 250 g/L, on the other hand, are notpreferred because a too high concentration of sodium hydroxide is notsuited for practical use.

1-3. Plating Accelerator

The plating accelerator to be contained in the plating bath of thepresent invention contains a heterocyclic compound represented by theabove chemical formula (I) and/or a derivative of the heterocycliccompound. The term “derivative of the heterocyclic compound representedby the above chemical formula (I)” as used herein means a derivative ofa 1,3,4-thiadiazole. Specific examples of the compound represented bythe above chemical formula (I) and derivatives thereof include2,5-dimercapto-1,3,4-thiadiazole, 2-thioaceticacid-5-mercapto-1,3,4-thiadiazole, 2,5-(dithioaceticacid)-1,3,4-thiadiazole,2-hydroxyethylthio-5-mercapto-1,3,4-thiadiazole,2,5-dihydroxyethylthio-1,3,4-thiadiazole, and epichlorohydrin-modified2,5-dimercapto-1,3,4-thiadiazole.

In FIG. 1, 2,5-dimercapto-1,3,4-thiadiazole is used as an example of theheterocyclic compound represented by the above chemical formula (I) orderivative thereof and it schematically illustrates an assumedadsorption state of 2,5-dimercapto-1,3,4-thiadiazole to a surface to beplated. As illustrated in FIG. 1, sulfur atoms constituting twosubstituents and a 5-membered ring chemically adsorb to the surface tobe plated. It is presumed that deposition of a metal such as zinc isaccelerated because a reduction reaction is likely to occur in a space,represented by a dotted pattern in FIG. 1, between the sulfur atomsconstituting two substituents and 5-membered ring and the surface to beplated. The above-described derivatives of the heterocyclic compoundrepresented by the chemical formula (I) embrace compounds having amolecular structure of a compound in which chemical adsorption as shownschematically in FIG. 1 is presumed to occur and available by adding asubstituent or carrying out a skeleton change based on the heterocycliccompound represented by the above chemical formula (I) without departingfrom the range permitting those skilled in the field of platingtechnology or organic chemistry to design.

The plating bath of the present invention contains the heterocycliccompound represented by the above chemical formula (I) and/or derivativethereof preferably in an amount of from 0.01 g/L or greater but notgreater than 1.0 g/L from the standpoint of improving the platingefficiency.

The above-described heterocyclic compounds represented by the aboveformula (I), derivatives thereof, and compounds prepared based on thesecompounds greatly contribute to an improvement in plating efficiencywhen electroplating is performed using the plating bath of the presentinvention. In addition, these heterocyclic compounds and derivativesthereof serve to bestow high brightness, in other words, give highbrightness to a zinc plating film formed using the plating bath of thepresent invention.

1-4. Primary Brightener:

The plating bath of the present invention contains a primary brightener.Examples of the primary brightener to be contained in the plating bathof the present invention include anionic surfactants, nonionicsurfactants, and water soluble macromolecular organic compounds such aspolyalkylene polyamines, each generally used for various zinc platingbaths as a primary brightener. Particularly in the plating bath of thepresent invention, primary brighteners containing a water solublecationic macromolecular compound are preferred.

Examples of the water soluble cationic macromolecular compound includepolyallylamines, polylamide polyamines, polyepoxy polyamines, andpolyalkylene polyamines. The polyallylamines include apolydiallyldimethylammonium chloride—sulfur dioxide copolymer. Thepolyepoxypolyamines include a condensation polymer of ethylenediamineand epichlorohydrin, a condensation polymer of dimethylaminopropylamineand epichlorohydrin, a condensation polymer of imidazole andepichlorohydrin, a condensation polymer of an imidazole derivative suchas 1-methylimidazole or 2-methylimidazole and epichlorohydrin, and acondensation polymer of a heterocyclic amine including a triazinederivative such as acetoguanamine or benzoguanamine and epichlorohydrin.The polyamide polyamines include polyamine polyurea resins, morespecifically, a condensation polymer of 3-dimethylaminopropylurea andepichlorohydrin and a condensation polymer ofbis(N,N-dimethylaminopropyl)urea and epichlorohydrin and water solublenylon resins, more specifically, a condensation polymer ofN,N-dimethylaminopropylamine, alkylenedicarboxylic acid, andepichlorohydrin. The polyalkylene polyamines include diethylenetriamine,triethylenetetramine, tetraethylenepentamine, hexamethylenepentamine, acondensation polymer of dimethylaminopropylamine and2,2′-dichlorodiethylether, a condensation polymer ofdimethylaminopropylamine and 1,3-dichloropropane, a condensation polymerof N,N,N′,N′-tetramethyl-1,3-diaminopropane and 2,2′-dichlorodiethylether, a condensation polymer ofN,N,N′,N′-tetramethyl-1,3-diaminopropane, and 1,4-dichlorobutane, and acondensation polymer of N,N,N′,N′-tetramethyl-1,3-diaminopropane and1,3-dichloropropane-2-ol.

In particular, the primary brightener contained in the plating bath ofthe present invention more preferably contains, as the water solublecationic macromolecular compound, a compound represented by thefollowing chemical formula (II) from the standpoint of improving theuniform electrodeposition and throwing power.

(In the chemical formula (II), n stands for a natural number of 1 orgreater, R¹, R², R³, and R⁴ may be the same or different and eachrepresents hydrogen, methyl, ethyl, isopropyl,2-hydroxylethyl-CH₂CH₂(OCH₂CH₂)_(x)OH (X stands for a natural numberfrom 0 to 6), or 2-hydroxyethyl-CH₂CH₂(OCCH₂CH₂)_(x)OH (X stands for anatural number from 0 to 6), R⁵ represents (CH₂)₂—O—(CH₂)₂,(CH₂)₂—O—(CH₂)—O—(CH₂)₂, or CH₂—CHOH—CH₂—O—CH₂—CHOH—CH₂, and Yrepresents S or O).

1-5: Secondary Brightener:

The plating bath of the present invention optionally contains asecondary brightener. Examples of the secondary brightener include thosecontaining an organic aldehyde and/or a heterocyclic compound.

Particularly from the standpoint of improving the brightness andimproving the throwing power, the secondary brightener to be used in theplating bath of the present invention preferably contains at leasteither one of an aromatic aldehyde or a pyridinium compound.

Examples of the aromatic aldehyde preferably contained as the secondarybrightener in the plating bath of the present invention includeanisaldehyde, veratraldehyde, salicylaldehyde, vanillin, piperonal, andp-hydroxybenzaldehyde. In particular, the aromatic aldehyde contained asthe secondary brightener to be used in the plating bath of the presentinvention is more preferably veratraldehyde and vanillin from thestandpoint of improvement of brightness and stability of the compound tobe contained in the zinc plating bath.

Examples of the pyridinium compound preferably contained as thesecondary brightener in the plating bath of the present inventioninclude benzyl pyridinium carboxylate (3-carboxybenzyl pyridiniumchloride) and 3-carbamoylbenzyl pyridinium chloride.

2. Embodiment for Forming a Zinc Alloy Plating Film

The plating bath of the present invention can be used not only for azinc plating film composed mainly of zinc but also for the formation ofa zinc alloy plating film such as zinc-nickel alloy (which willhereinafter be called “Zn—Ni alloy”), zinc-iron alloy (which willhereinafter be called “Zn—Fe alloy”), and zinc-iron-nickel alloy (whichwill hereinafter be called “Zn—Fe—Ni alloy”) by electroplating.Incidentally, when nickel ions functioning also as a metal brightenerare incorporated in the plating bath of the present invention, a zincplating film (Zn—Ni alloy film or Zn—Fe—Ni alloy film) having a goodbrightness appearance can sometimes be formed without the secondarybrightener (refer to Example 8 which will be described later).

In the plating bath of the present invention, nickel ions may beincorporated for the formation of a Zn—Ni alloy plating film; iron ionsmay be incorporated for the formation of a Zn—Fe alloy plating film; andiron ions and nickel ions may be incorporated for the formation of aZn—Fe—Ni alloy plating film. When the nickel ions are incorporated inthe plating bath of the present invention, it is possible to dissolve,for example, NiSO₄•6H₂O, NiCl₂•6H₂O, Ni(OH)₂, or the like in the platingbath. When iron ions are incorporated in the plating bath of the presentinvention, it is possible to dissolve, for example, Fe₂(SO₄)₃•7H₂O,FeSO₄•7H₂O, Fe(OH)₃, FeCl₃•6H₂O, FeCl•4H₂ O or the like.

When the plating bath of the present invention contains nickel ions, anamount of nickel ions is preferably 100 mg/L or greater but not greaterthan 4000 mg/L from the standpoint of improving the qualities of thezinc plating film (appearance, film thickness, ease of after treatment,corrosion resistance). When the plating bath contains the nickel ions inan amount less than 100 mg/L or exceeding 4000 mg/L, a zinc plating filmsuperior in desired appearance, ease of after treatment, ease ofchemical conversion treatment, corrosion resistance, or heat resistancecannot be obtained.

When the plating bath of the present invention contains iron ions, anamount of iron ions is preferably from 10 mg/L or greater but notgreater than 150 mg/L from the standpoint of improving the quality ofthe zinc plating film (appearance, film thickness, ease of aftertreatment, corrosion resistance). When the plating bath contains ironions in an amount less than 10 mg/L or exceeding 150 mg/L, a zincplating film superior in desired appearance, ease of after treatment,ease of chemical conversion treatment, corrosion resistance, or heatresistance cannot be obtained.

In the embodiment for forming a zinc alloy plating film, incorporationof a chelating agent in the plating bath of the present invention ispreferred. As this chelating agent, a primary brightener alsofunctioning as a chelating agent such as diethylenetriamine ortriethylenetetramine may be employed. In the embodiment for forming azinc alloy plating film having, as a component thereof, nickel, iron, orboth of them, the plating bath of the present invention contains, as achelating agent, preferably tartaric acid, citric acid, gluconic acid,ethylenediamine, hexamine, 1,2-diaminopropane, 1,3-diaminopropane,1,2-diaminobutane, 1,4-diaminobutane, triaminotriethylamine,methylaminopropylamine, monoethanolamine, diethanolamine,triethanolamine, diethanolaminopropylamine,2-hydroxyethylaminopropylamine, 1,3-bis(3-aminopropoxy)ethane,nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA),diethylenetriaminepentaacetic acid (DTPA),triethylenetetraminehexaacetic acid (TTHA), or the like.

3. Action/Effect of the Zincate Zinc Plating Bath of the PresentInvention:

FIG. 2 shows the relationship between current density and platingefficiency when electroplating causing a wide range of current densitydistribution is performed with the conventionally known zincate zincplating bath (including known improved ones prior to the present patentapplication) and the zincate zinc plating bath of the present invention.First, the conventional zincate zinc plating bath has a high platingefficiency at a high current density, while it has a low platingefficiency at a low current density (a broken line A in FIG. 2). Suchvariations in plating efficiency due to a difference in the magnitude ofcurrent density occurs because deposition of zinc is accelerated only ata high current density. In addition, zinc ions are consumed at aposition of a high current density and sufficient zinc ions fordeposition cannot be ensured at a position of a low current density.This increases the variation in plating efficiency due to a differencein the magnitude of current density. In an improved zincate zinc platingbath which is known prior to the present patent application and containsa water soluble cationic macromolecular compound represented by thebelow chemical formula (III) as a primary brightener, the platingefficiency at a high current density is suppressed by reducing thecurrent efficiency at a high current density. Thus, in the aboveimproved zincate zinc plating bath, a difference in plating efficiencybetween a high current density and a low current density decreases anduniform electrodeposition is improved, but an average plating efficiencyis low and a zinc plating film thus formed is thin (a broken line B inFIG. 2). In the plating bath of the present invention, a platingefficiency at a low current density is enhanced (a solid line C in FIG.2, for details, refer to Evaluation test 4 in Example which will bedescribed later). This improvement in plating efficiency at a lowcurrent density is mainly caused by improvement in uniformelectrodeposition in the plating bath of the present invention. With theplating bath of the present invention, an average plating efficiency ishigh and a thick and a uniform zinc plating film can be formed (fordetails, refer to Evaluation test 3 in Example which will be describedlater).

EXAMPLES

The present invention will hereinafter be described in further detailbased on Examples. It should however be noted that the present inventionis not limited to or by them.

4. Preparation of Zincate Zinc Plating Bath:

4-1. For Zinc Plating Film Having Only Zinc as a Metal Component:

Example 1

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 8 g/L of zinc ions, 10 g/L of sodium hydroxide,0.4 g/L of a 1:1 (molar ratio) condensation polymer ofdimethylaminopropylamine and epichlorohydrin (Compound A of Table 1,product of URSA CHEMIE GMBH, Product number: Product J 138 M) as aprimary brightener, 0.03 g/L of veratraldehyde—Na sulfite adduct(product of Wako Pure Chemical Industries, which will equally applyhereinafter) as a secondary brightener obtained by dissolving andreacting in a sodium hydrogen sulfite solution, and 0.05 g/L of2,5-dimercapto-1,3,4-thiadiazole (Compound J in Table 1, product of ToyoChemical Industry, MTD) as a plating accelerator, was prepared. Theconcentration of each of the above compounds (including a polymer) wasadjusted by conversion in terms of a purity (content) and a molecularweight of a commercially available product of each compound obtained(which will equally apply to determination of the concentration of acompound of another commercially available product).

TABLE 1 Secondary brightener Sodium Primary brightener Benzyl Platingaccelerator Zinc Nickel hydroxide Kind *1 Anisaldehyde Veratraldehydepyridinium Kind *1 ions (g/L) ions (g/L) (g/L) [Concentration (g/L)](g/L) (g/L) carboxylate (g/L) [Concentration (g/L)] Example 1 8 0 100 A[0.4] 0 0.03 0 J [0.050] Example 2 6 0 100 B [1.0] 0.025 0 0 J [0.020]Example 3 9 0 120 B [0.6] 0 0.025 0 J [0.075] Example 4 12 0 120 B [1.0]0.025 0 0 J [0.100] Example 5 15 0 135 B [1.5] 0.050 0 0 J [0.200]Example 6 12 0 125 C [0.4] 0 0 0.06 J [0.050] Example 7 13 0 130 D [1.5]0 0.03 0 J [0.100] Example 8 11 2.25 120 E [30] 0 0 0 J [0.080] Example9 10 0 100 B [1.0] 0.03 0 0 J [0.060] Example 10 18 0 140 B [1.2] + F[1.0] *2 0.09 0 0 J [0.050] Example 11 16 0 140 B [0.6] + G [1.5] 0.03 00 K [0.100] Example 12 13 0 130 G [1.2] + H [0.15] 0 0 0.03 L [0.075]Example 13 12 0 120 B [1.2] + I [0.5] 0.09 0 0 M [0.200] Comp. Ex. 1 8 0100 A [1.0] 0 0.03 0 None Comp. Ex. 2 10 0 100 B [1.0] 0.03 0 0 NoneComp. Ex. 3 14 0 140 B [1.2] + F [0.75] 0.03 0 0 None *1: A to Drepresent the following compounds. A: condensation polymer ofdimethylaminopropylamine and epichlorohydrin B: compound represented bythe chemical formula (III) (MIRAPOL WT) C: polydiallyldimethylammoniumchloride/sulfur dioxide copolymer D: a 1:2 (molar ratio) condensationpolymer of ethylenediamine and epichlorohydrin E: tetraethylenepentamineF: a 1:0.73 (molar ratio) condensation polymer of imidazole andepichlorohydrin (containing about 18% of an imidazole component) G: a1:1 (molar ratio) condensation polymer of imidazole and epichlorohydrin(containing about 18% of an imidazole component) H: a 1:0.73 (molarratio) condensation polymer of 1-methylimidazole and epichlorohydrin(containing about 21% of a 1-methylimidazole component) I: a 1:1 (molarratio) condensation polymer of 2-methylimidazole and epichlorohydrin(containing about 21% of a 2-methylimidazole component) J:2,5-dimercapto-1,3,4-thiadiazole K: 2-(thioaceticacid)-5-mercapto-1,3,4-thiadiazole L:2-hydroxyethylthio-5-mercapto-1,3,4-thiadiazole M:epichlorohydrin-modified 2,5-dimercapto-1,3,4-thiadiazole *2: Itcontains 1.2 (g/L) of Compound B as described above in *1 and 1.0 (g/L)of Compound F as a primary brightener (this also applies to Examples 11to 13, and Comparative Example 3).

Example 2

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 6 g/L of zinc ions, 100 g/L of sodium hydroxide,1.0 g/L of a water soluble cationic macromolecular compound representedby the below-described formula (III) (Compound B of Table 1, trade name:MIRAPOL (trade mark) WT, product of Rhodia [formerly Rhone-Poulence],CAS No. 68555-36-2) as a primary brightener, 0.025 g/L ofanisaldehyde—Na sulfite adduct (product of Wako Pure ChemicalIndustries, which will equally apply hereinafter) as a secondarybrightener obtained by dissolving and reacting in a sodium hydrogensulfite solution, and 0.02 g/L of 2,5-dimercapto-1,3,4-thiadiazole(Compound J of Table 1) as a plating accelerator was prepared.

Example 3

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 9 g/L zinc ions, 120 g/L sodium hydroxide, 0.6g/L of a water soluble cationic macromolecular compound represented bythe above chemical formula (III) (Compound B of Table 1, trade name:MIRAPOL (trade mark) WT, (product of Rhodia), CAS No. 68555-36-2) as aprimary brightener, 0.025 g/L of veratraldehyde as a secondarybrightener, and 0.075 g/L of 2,5-dimercapto-1,3,4-thiadiazole (CompoundJ of Table 1) as a plating accelerator was prepared.

Example 4

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 12 g/L of zinc ions, 120 g/L of sodiumhydroxide, 1.0 g/L of a water soluble cationic macromolecular compoundrepresented by the above-described formula (III) (Compound B of Table 1,trade name: MIRAPOL (trade mark) WT, product of Rhodia, CAS No.68555-36-2) as a primary brightener, 0.025 g/L of an anisaldehyde—Nasulfite adduct as a secondary brightener, and 0.100 g/L of2,5-dimercapto-1,3,4-thiadiazole (Compound J of Table 1) as a platingaccelerator was prepared.

Example 5

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 15 g/L of zinc ions, 135 g/L of sodiumhydroxide, 1.5 g/L of a water soluble cationic macromolecular compoundrepresented by the above-described formula (III) (Compound B of Table 1,trade name: MIRAPOL (trade mark) WT, product of Rhodia, CAS No.68555-36-2) as a primary brightener, 0.050 g/L of an anisaldehyde—Nasulfite adduct as a secondary brightener, and 0.200 g/L of2,5-dimercapto-1,3,4-thiadiazole (Compound J of Table 1) as a platingaccelerator was prepared.

Example 6

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 12 g/L of zinc ions, 125 g/L of sodiumhydroxide, 0.4 g/L of polydiallyldimethylammonium chloride/sulfurdioxide copolymer (Compound C of Table 1, trade name:diallyldimethylammonium chloride—sulfur dioxide copolymer [quaternaryamine/SO₂], product of Nitto Boseki, Product No: PAS-A-5 as a primarybrightener, 0.06 g/L of benzyl pyridinium carboxylate (benzyl pyridiniumcarboxylate aqueous solution, Lugalvan BPC-48, product of BASF) as asecondary brightener, and 0.050 g/L of 2,5-dimercapto-1,3,4-thiadiazole(Compound J of Table 1) as a plating accelerator was prepared.

Example 7

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 13 g/L of zinc ions, 130 g/L of sodiumhydroxide, 1.5 g/L of a condensation polymer obtained by reactingethylene diamine and epichlorohydrin at a molar ratio of 1:2 (Compound Dof Table 1, product of URSA CHEMIE GMBH, product No. Product JF 220) asa primary brightener, 0.03 g/L of a veratraldehyde—Na sulfite adduct asa secondary brightener, and 0.100 g/L of2,5-dimercapto-1,3,4-thiadiazole (Compound J of Table 1) as a platingaccelerator was prepared.

Example 9

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 10 g/L of zinc ions, 100 g/L of sodiumhydroxide, 1.0 g/L of a water soluble cationic macromolecular compoundrepresented by the above chemical formula (III) (Compound B of Table 1,trade name: MIRAPOL (trade mark) WT, product of Rhodia, CAS No.68555-36-2) as a primary brightener, 0.03 g/L of an anisaldehyde—Nasulfite adduct as a secondary brightener, and 0.060 g/L of2,5-dimercapto-1,3,4-thiadiazole (Compound J of Table 1) as a platingaccelerator was prepared.

Example 10

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 18 g/L of zinc ions, 140 g/L of sodiumhydroxide, 1.2 g/L of a water soluble cationic macromolecular compoundrepresented by the above chemical formula (III) (Compound B of Table 1,trade name: MIRAPOL (trade mark) WT, product of Rhodia, CAS No.68555-36-2) and 1.0 g/L, in terms of a solid content, of a condensationpolymer obtained by reacting imidazole and epichlorohydrin at a molarratio of 1:0.73 (containing approximately 18% of the imidazolecomponent) (Compound F of Table 1) as a primary brightener, 0.09 g/L ofan anisaldehyde—Na sulfite adduct as a secondary brightener, and 0.050g/L of 2,5-dimercapto-1,3,4-thiadiazole (Compound J of Table 1) as aplating accelerator was prepared.

Example 11

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 16 g/L of zinc ions, 140 g/L of sodiumhydroxide, 0.6 g/L of a water soluble cationic macromolecular compoundrepresented by the above chemical formula (III) (Compound B of Table 1,trade name: MIRAPOL (trade mark) WT, product of Rhodia, CAS No.68555-36-2) and 1.5 g/L of a condensation polymer obtained by reactingimidazole and epichlorohydrin at a molar ratio of 1:1 (containingapproximately 18% of the imidazole component) (Compound G of Table 1) asa primary brightener, 0.03 g/L of an anisaldehyde—Na sulfite adduct as asecondary brightener, and 0.100 g/L of 2-(thioaceticacid)-5-mercapto-1,3,4-thiadiazole (Compound K of Table 1) as a platingaccelerator was prepared.

Example 12

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 13 g/L of zinc ions, 130 g/L of sodiumhydroxide, 1.2 g/L of a water soluble cationic macromolecular compoundrepresented by the above chemical formula (III) (Compound B of Table 1,trade name: MIRAPOL (trade mark) WT, product of Rhodia, CAS No.68555-36-2) and 0.15 g/L of a condensation polymer obtained by reacting1-methylimidazole (product of Nippon Synthetic Chemical Industry, 1MI)and epichlorohydrin at a molar ratio of 1:073 (containing approximately21% of the 1-methylimidazole component) (Compound H of Table 1) as aprimary brightener, 0.03 g/L of an aqueous solution of benzyl pyridiniumcarboxylate (product of BASF, Lugalvan BPC-48) as a secondarybrightener, and 0.075 g/L of2-hydroxyethylthio-5-mercapto-1,3,4-thiadiazole (Compound L of Table 1)as a plating accelerator was prepared.

Example 13

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 12 g/L of zinc ions, 120 g/L of sodiumhydroxide, 1.2 g/L of a water soluble cationic macromolecular compoundrepresented by the above chemical formula (III) (Compound B of Table 1,trade name: MIRAPOL (trade mark) WT, product of Rhodia, CAS No.68555-36-2) and 0.5 g/L of a condensation polymer obtained by reacting2-methylimidazole (product of Nippon Synthetic Chemical Industry, 2MI)and epichlorohydrin at a molar ratio of 1:1 (containing approximately21% of the 2-methylimidazole component) (Compound I of Table 1) as aprimary brightener, 0.09 g/L of an anisaldehyde—Na sulfite adduct as asecondary brightener, and 0.20 g/L of epichlorohydrin-modified2,5-dimercapto-1,3,4-thiadiazole (Compound M of Table 1) as a platingaccelerator was prepared.

Comparative Example 1

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 8 g/L of zinc ions, 100 g/L of sodium hydroxide,1.0 g/L of a 1:1 (molar ratio) condensation polymer ofdimethylaminopropylamine and epichlorohydrin (Compound A of Table 1,product of URSA CHEMIE GMBH, product No: Product J 138 M) as a primarybrightener, and 0.03 g/L of an veratraldehyde—Na sulfite adduct as asecondary brightener was prepared.

Comparative Example 2

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 10 g/L of zinc ions, 100 g/L of sodiumhydroxide, 1.0 g/L of a water soluble cationic macromolecular compoundrepresented by the above chemical formula (III) (Compound B of Table 1,trade name: MIRAPOL (trade mark) WT, product of Rhodia, CAS No.68555-36-2) as a primary brightener, and 0.03 g/L of an anisaldehyde—Nasulfite adduct as a secondary brightener was prepared.

Comparative Example 3

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 14 g/L of zinc ions, 140 g/L of sodiumhydroxide, 1.2 g/L of a water soluble cationic macromolecular compoundrepresented by the above chemical formula (III) (Compound B of Table 1,trade name: MIRAPOL (trade mark) WT, product of Rhodia, CAS No.68555-36-2) and 0.75 g/L of a condensation polymer obtained by reactingimidazole and epichlorohydrin at a molar ratio of 1:0.73 (containingapproximately 18% of the imidazole component) (Compound F of Table 1) asa primary brightener, and 0.03 g/L of an anisaldehyde—Na sulfite adductas a secondary brightener was prepared.

4-2. For Zinc-Nickel Alloy Plating Film:

Example 8

In accordance with the composition shown in Table 1, a zincate zincplating bath containing 11 g/L of zinc ions, 2.25 g/L of nickel ions,120 g/L of sodium hydroxide, 30 g/L of tetraethylenepentamine (CompoundE of Table 1) as a primary brightener, and 0.080 g/L of2,5-dimercapto-1,3,4-thiadiazole (Compound J of Table 1) as a platingaccelerator was prepared.

5. Evaluation Test of Zincate Zinc Plating Bath:

Using the zincate zinc plating baths of Examples 1 to 13 and ComparativeExamples 1 to 3, electroplating was performed in accordance with eachevaluation item described below and the zinc plating films thus formedwere evaluated.

5-1. Evaluation Test (Evaluation Test 1) of Brightness Appearance,Peeling, and Blister of a Zinc Plating Film Formed by ElectroplatingCausing a Wide Range of Current Density Distribution Including a CurrentDensity Region (from 0.05 to 2.5 A/dm²):

5-1-1: Electroplating:

Each of the plating baths of Examples 1 to 13 and Comparative Examples 1to 3 was placed in an circulating type hull cell (trade name: Smart HullCell, product of Yamamoto MS, product No: B-53-SM) equipped with a stirhaving a rotation speed of 1000 rpm and electroplating was performed. Asan anode and a cathode for the formation of a zinc plating film, an ironsheet of 45 mm long, 45 mm wide, and 1 mm thick and an iron sheet of 67mm long, 100 mm wide, and 0.3 mm thick made by Yamamoto MS were used,respectively. Electroplating was performed under the followingconditions: current of 0.5 A (current density of from 0.05 to 2.5A/dm²), current carrying time of 60 minutes, and electroplating bathtemperature of 25° C.

5-1-2. Results of Evaluation Test of Brightness Appearance andEvaluation:

Table 2 shows the results of the evaluation test conducted for visuallyjudging the brightness of the zinc plating films obtained using thezincate zinc plating baths of Examples 1 to 13 and Comparative Examples1 to 3. As a result of comparison between Example 2 and ComparativeExample 2 which are contrast examples from the standpoint of thepresence or absence of 2,5-dimercapto-1,3,4-thiadiazole (Compound J ofTable 1) contained as a plating accelerator, the zinc plating filmformed in Example 2 has an entire brightness, while the zinc platingfilm formed in Comparative Example 2 has a semi-brightness. Thissuggests that 2,5-dimercapto-1,3,4-thiadiazole contained as a platingaccelerator acts to improve the brightness of the zinc plating film.Similarly, 2-(thioacetic acid)-5-mercapto-1,3,4-thiadiazole (Compound Kof Table 1) contained in Example 11,2-hydroxyethylthio-5-mercapto-1,3,4-thiadiazole (Compound L of Table 1)contained in Example 12, and epichlorohydrin-modified2,5-dimercapto-1,3,4-thiadiazole (Compound M of Table 1) contained inExample 13, each as a plating accelerator, also acts to improve thebrightness of the respective zinc plating films thus obtained. The term“entire brightness” in Table 2 means that a plating film hassubstantially a uniform mirror surface. The term “brightness” in Table 2means that a plating film has an entire brightness but the brightness isa little dull. The term “semi-brightness” means that a plating film hasmore dull brightness. The term “entire white” means that a plating filmhas a uniformly matte surface.

TABLE 2 Evaluation test 1 Evaluation test 2 Evaluation test 4 Evaluationtest 5 Brightness Peeling • Blackness • Plating efficiency A percentageto the plating Uniform appearance blister brightness (mg/A · min)efficiency of Com. Ex. 1 (%) electrodeposition Example 1 Entirebrightness None Good 89.3 123 47.9 Example 2 Entire brightness None Good87.3 120 66.3 Example 3 Entire brightness None Good 95.3 131 69.0Example 4 Entire brightness None Good 92.8 128 65.6 Example 5 BrightnessNone Good 84.3 116 61.5 Example 6 Semi-brightness None Lack ofbrightness 92.3 127 35.6 Example 7 Entire white None Lack of brightness87.0 120 47.8 Example 8 Entire brightness None Good — — — Example 9Entire brightness None Good 92.8 128 63.0 Example 10 Entire brightnessNone Good 92.5 128 77.0 Example 11 Entire brightness None Good 90.0 12475.5 Example 12 Entire brightness None Good 88.9 123 80.0 Example 13Entire brightness None Good 92.0 127 67.0 Comp. Ex. 1 Entire brightnessNone Gas trace defect 72.5 100 30.7 Comp. Ex. 2 Semi-brightness NoneLack of brightness 77.6 107 60.8 Comp. Ex. 3 Brightness None Lack ofbrightness 82.0 113 65.0

5-1-3. Results of Evaluation Test of Peeling and Blister and Evaluation:

For the evaluation test of peeling and blister of a zinc plating film,rightly after electroplating using the above-described circulating typehull cell, the plated materials obtained using the plating baths ofExamples and Comparative Examples were each heat treated at 60° C. for72 hours. Table 2 shows the results of the evaluation test conducted byvisually confirming the presence or absence, after the heat treatment,of peeling and blister of the zinc plating films formed using thezincate zinc plating baths of Examples 1 to 13 and Comparative Examples1 to 3. Neither peeling nor blister was observed from the heat-treatedzinc plating films obtained using the baths of these Examples andComparative Examples.

Incidentally, a zinc plating film was formed even on the surface of theiron sheet of the cathode, which was on the opposite side to a surfacefacing the anode, as a result of electroplating using the zincate zincplating baths of Examples 1 to 13. This fact has revealed that thezincate zinc plating baths of Examples 1 to 13 have an excellentthrowing power enough for forming a zinc plating film by utilizing avery faint current leaked to the reverse side of the iron sheet of thecathode.

5-2. Evaluation Test on the Brightness and the Like of a Plating FilmObtained by Subjecting the Zinc Plating Film Formed Using a BarrelPlating to Chemical Conversion Treatment (Evaluation Test 2):

5-2-1. Barrel Plating and Chemical Conversion Treatment:

Each of the zincate zinc plating baths of Examples 1 to 13 andComparative Examples 1 to 3 was placed in a minibarrel (diameter [Ø] ofa drum×length [L]: 110 mm×150 mm) and 1 kg of a hexagon bolt made ofiron having a size, in accordance with JIS standards, of M10 wassubjected to barrel plating at a current of 5A and a current carryingtime of 50 minutes. After the barrel plating, the plated product washeat treated at 200° C. for 3 hours and then allowed to cool, followedby chemical conversion treatment at 35° C. for 45 seconds at pH 2.2 witha trivalent chromium black chemical conversion treatment agent (tradename: Metasu YFB-SA/SB, product of Yuken Industry). Then, the platedproduct was subjected to finishing treatment with a finishing treatmentagent (trade name: Metasu CR-U3/I2, product of Yuken Industry) and thendried at 80° C. for 20 minutes. Thus, barrel plating was performed witheach of the zincate zinc plating baths of Examples 1 to 13 andComparative Examples 1 to 3, followed by chemical conversion treatmentto form zinc plating films (hereinafter called “chemical conversiontreatment zinc plating film”).

5-2-2. Results and Evaluation:

Table 2 shows the results of the evaluation test by which blackness andbrightness of the chemical conversion treatment zinc plating filmsobtained using the zincate zinc plating baths of Examples 1 to 13 andComparative Examples 1 to 3 were confirmed visually. The chemicalconversion treatment zinc plating films formed in Examples 1 to 5 andExamples 8 to 13 showed good results in blackness and brightness. Thechemical conversion treatment zinc plating films formed in Examples 6and 7 showed good results in blackness, but their brightness was notsufficiently good (the definition of “brightness not sufficiently good”will be described later). The chemical conversion treatment zinc platingfilm formed in Comparative Example 1, which is a contrast example ofExample 1 from the standpoint of presence or absence of2,5-dimercapto-1,3,4-thiadiazole (Compound J of Table 1) contained as aplating accelerator, caused a trace defect due to generation of a gas.The chemical conversion treatment zinc plating film formed inComparative Example 2, which is a contrast example of Example 2 from thestandpoint of a plating accelerator, showed good results in blacknessbut it lacked brightness. It has therefore been elucidated from theresults of Examples 1 to 5 and 8 to 13 that in the practical embodimentusing barrel plating and conversion treatment in combination, thezincate zinc plating baths containing Compounds J, K, L, and M of Table1, respectively, as a plating accelerator have also an enhancing actionof qualities (appearance, ease of after treatment) of a zinc platingfilm. Incidentally, the term “good results in blackness” in thisevaluation test means that the zinc plating film is jet black. The term“good results in brightness” in this evaluation test means that the zincplating film has a gloss. The term “brightness not sufficiently good” inthis evaluation test means that the zinc plating film lacks brightnessand is smoky. The term “gas trace defect” in this evaluation test meansthat the zinc plating film has a white flow trace due to generation of ahydrogen gas.

5-3. Evaluation Test on the Relationship Between Current DensityDistribution and Thickness of Zinc Plating Film (Evaluation Test 3)

5-3-1: Electroplating

The plating bath of Example 9 was placed in an circulating type hullcell (trade name: Smart Hull Cell, product of Yamamoto MS, product No:B-53-SM) equipped with a stir having a rotation speed of 1000 rpm andelectroplating was performed. As an anode and a cathode for theformation of a zinc plating film, an iron sheet of 45 mm long, 45 mmwide, and 1 mm thick and an iron sheet of 67 mm long, 100 mm wide, and0.3 mm thick made by Yamamoto MS were used, respectively. Electroplatingwas performed under the following conditions: current of 1.0 A, currentcarrying time of 15 minutes, and electroplating bath temperature of 30°C.

5-3-2. Measurement of Thickness of Zinc Plating Film:

Thicknesses of the zinc plating film at five positions, that is, 10 mm,40 mm, 60 mm, 80 mm, and 90 mm from the end of the high current portiontoward the end of the low current portion at the center of the platedsurface of the hull cell cathode sheet were measured using a fluorescentX-ray coating thickness gauge (product of SII, SFT-9200). Thesepositions correspond to 5.0 A/dm², 2.0 A/dm², 1.0 A/dm², 0.33 A/dm², and0.1 A/dm², respectively, in primary current distribution theoreticallyderived from the electroplating conditions at a current of 1.0 A in theabove-described hull cell form.

5-3-3. Results and Evaluation:

FIG. 3 shows the relationship between the current density distributionand the thickness of a zinc plating film. In FIG. 3, the theoreticallyderived current density distribution is indicated by a blank circle (◯)and the thickness of the zinc plating film derived theoretically fromthis theoretic value of the current density is indicated by a blanktriangle (Δ). In FIG. 3, a solid line indicates a film thickness and abroken line indicates a current density. In FIG. 3, measured values ofthe thickness of the zinc plating film in the present test using theplating bath of Example 9 is plotted with a solid triangle. Thethicknesses of the zinc plating film at the points where the theoreticalcurrent densities were 0.5 A/dm², 2.0 A/dm², 1.0 A/dm², 0.33 A/dm², and0.1 A/dm², respectively were all around 6 μm. This suggests that auniform and thick zinc plating film can be formed using the plating bathof Example 9 even in electroplating causing a current densitydistribution as wide as from 0.1 to 5.0 A/dm². The results also showthat a uniform and thick zinc plating film is formed even by carryingout electroplating for a current carrying time as short as 15 minutes inthe plating both of Example 9. Incidentally, when a conventionally knownzincate zinc plating bath is used, a zinc plating film having an averagethickness of at most approximately from 3 to 4 μm can be formed even acurrent carrying time is increased (data is not shown). In the presenttest using the plating bath of Example 9, the average current densitycalculated from the measured film thickness was as low as 1.5 A/dm²(plotted with a solid square (▪) in FIG. 3. Refer to Evaluation Test 4with regards to the evaluation of plating efficiency at an averagecurrent density (1.0 A/dm²).

5-4. Evaluation Test on Plating Efficiency at Average Current Density(1.0 A/dm²) (Evaluation Test 4):

5-4-1. Electroplating:

Zinc plating films were formed on 0.25 dm² in total of both surfaces ofa cold rolled steel plate (SPCC) 50 mm long, 25 mm wide, and 0.5 mmthick by electroplating using the zincate zinc plating baths of Examples1 to 7 and 9 to 13, and Comparative Examples 1 to 3. The electroplatingwas performed under the conditions of average current density of 1.0A/dm², current carrying time of 20 minutes, and plating bath temperatureof 25° C.

5-4-2. Calculation of Plating Efficiency:

The zinc plating films formed using the zincate zinc plating baths ofExamples 1 to 7 and 9 to 13, and Comparative Examples 1 to 3 were eachdissolved completely in dilute hydrochloric acid to prepare an analysissample. The analysis sample was subjected to IPC analysis to find thezinc concentration therein. Based on the zinc concentration of theanalysis sample, a zinc content in the zinc plating film, that is, adeposition amount of zinc due to electroplating was calculated. 2plating efficiency (mg/A•min) was determined by dividing the zincdeposition amount of each of the zinc plating films formed using theplating baths of Examples 1 to 7 and 9 to 13, and Comparative Examples 1to 3 by a current and current carrying time of electroplating.

5-4-3. Results and Evaluation

Table 2 shows the results of the evaluation test of the plating baths ofExamples 1 to 7 and 9 to 13, and Comparative Examples 1 to 3 on platingefficiency and a ratio (percentage) to the plating efficiency ofComparative Example 1. As a result of a comparison between Example 1 andComparative Example 1, Example 2 or 9 and Comparative Example 2, andExample 10 and Comparative Example 3, which are each a contrasting pairfrom the standpoint of the presence or absence of2,5-dimercapto-1,3,4-thiadiazole (Compound J of Table 1) contained in aplating accelerator, the baths of Examples 1, 2, 9, and 10 containing2,5-dimercapto-1,3,4-thiadiazole as a plating accelerator exhibit abetter plating efficiency than those of Comparative Examples 1 to 3. Inparticular, the baths of Examples 1 to 7 and 9 to 13 containing CompoundJ, K, L, or M shown in Table 1 as a plating accelerator showed a platingefficiency as high as 84 or greater. It has therefore been elucidatedthat 2,5-dimercapto-1,3,4-thiadiazole contained as a plating acceleratoris effective for enhancing the plating efficiency at a current densityof 1.5 A/dm² or less.

Examples 1 to 7, 9, and 10 using 2,5-dimercapto-1,3,4-thiadiazole(Compound J of Table 1), Example 11 using 2-(thioaceticacid)-5-mercapto-1,3,4-thiadiazole (Compound K of Table 1), Example 12using 2-hydroxyethylthio-5-mercapto-1,3,4-thiadiazole (Compound L ofTable 1), and Example 13 using epichlorohydrin-modified2,5-dimercapto-1,3,4-thiadiazole (Compound M of Table 1), each as aplating accelerator, showed a significant increase, that is, a 16 to 31%increase in plating efficiency compared with that of Comparative Example1.

In plating baths containing a water soluble cationic macromolecularcompound (MIRAPOL (trade mark) WT) represented by the above formula(III) at a low average current density of 1.0 A/dm², the 16 to 31%plating-efficiency increase brought by the plating baths of Examples 1to 7 and 9 to 13 is very significant compared with the 7% and 13%plating-efficiency increases brought by the plating baths of ComparativeExamples 2 and 3 belonging to the conventional art.

5-5. Evaluation Test (Evaluation Test 5) on Uniform Electrodeposition:

5-5-1: Electroplating

The plating baths of Examples 1 to 7 and 9 to 13, and ComparativeExamples 1 to 3 were each placed in an circulating type hull cell (tradename: Smart Hull Cell, product of Yamamoto MS, product No: B-53-SM)equipped with a stir having a rotation speed of 1000 rpm andelectroplating was performed. As a cathode, an iron sheet 67 mm long,100 mm wide, and 0.3 mm thick made by Yamamoto MS was used.Electroplating was performed under the following conditions: current of0.5 A (current density from 0.05 to 2.5 A/dm², current carrying time of60 minutes, and plating bath temperature of 25° C.

5-5-2. Measurement of Thickness of Zinc Plating Film:

The thickness of the zinc plating film was measured at below-describedtwo points of the above iron sheet used as the cathode. The positionwhich was on the surface of the iron sheet, at the center of the surfaceto be plated in a depth direction of a plating cell when it was placedin a hull cell (plating cell), and which was 10 mm from a high currentdensity end toward a low current density end Was designated as Measuringpoint A. The position which was on the same surface of the same ironsheet, at the same depth as Measuring point A, and which was 85 mm froma high current density end toward a low current density end wasdesignated as Measuring point B. The thicknesses of the zinc platingfilms at Measuring points A and B were measured using a fluorescentX-ray film thickness gauge (SFT-9200, product of SII). Uniformelectrodeposition was determined as a ratio (percentage) of the filmthickness of the zinc plating film at Measuring point B to that atMeasuring point A.

5-5-3. Results and Evaluation:

Table 2 shows the results of the evaluation test on uniformelectrodeposition of the zincate zinc plating baths of Examples 1 to 7and 9 to 13, and Comparative Examples 1 to 3. Example 1 and ComparativeExample 1 and Example 2 and Comparative Example 2 which form acontrasting pair from the standpoint of the presence or absence of2,5-dimercapto-1,3,4-thiadiazole (Compound J of Table 1) contained inthe plating accelerator are examined. The value of uniformelectrodeposition determined by the above calculation method is 47.9 inExample 1, while it is 30.7 in Comparative Example 1. Similarly, thevalue of uniform electrodeposition is 66.3 in Example 2, while it is60.8 in Comparative Example 2. Comparison of them has revealed that azincate zinc plating bath containing 2,5-dimercapto-1,3,4-thiadiazole ina plating accelerator exhibits improved uniform electrodeposition.

The plating baths of Examples 2 to 5 and 9 to 13 and Comparative Example2 containing a water soluble cationic macromolecular compound (MIRAPOL(trade mark) WT) represented by the above chemical formula (III) as aprimary brightener exhibited uniform electrodeposition as high as 60 orgreater. This result has revealed that the water soluble cationicmacromolecular compound (MIRAPOL (trade mark) WT) represented by theabove chemical formula (III) contained as a primary brightener issignificantly effective for the improvement of uniformelectrodeposition. The plating baths of Examples 2 to 5 and 9 to 13containing any of Compounds J to M of Table 1 as a plating acceleratorexhibited higher uniform electrodeposition. It has therefore beenelucidated that the above plating accelerators are effective forimproving uniform electrodeposition further. In particular, the platingbaths of Examples 10 to 13 using the water soluble cationicmacromolecular compound (MIRAPOL (trade mark) WT) represented by theabove chemical formula (III) and a condensation polymer of imidazole andepichlorohydrin (any of Compounds F to I of Table 1) in combination andcontaining any of Compounds J to M of Table 1 as a plating acceleratorshowed uniform electrodeposition as high as 67 to 80.

5-6 Synthetic Evaluation:

The above evaluation results have experimentally shown that the platingbaths of Examples 1 to 13, that is, the plating baths of the presentinvention enable the formation of a zinc plating film having highbrightness at a high plating efficiency even in electroplating at a lowaverage current density (form 0.05 to 1.0 A/dm²). The zincate zincplating baths of Examples 2 to 5 and 9 to 13 are embodiments of theplating bath of the present invention containing, as a platingaccelerator, at least any one of Compounds J to M of Table 1corresponding to the heterocyclic compound represented by the abovechemical formula (I) and a water soluble cationic macromolecularcompound (MIRAPOL (trade mark) WT) represented by the above chemicalformula (III) as a primary brightener. It has been shown experimentallythat the above-mentioned plating baths show high uniformelectrodeposition in addition to a high plating efficiency at a lowaverage current density. It has also been shown experimentally that thezincate zinc plating baths of Examples 1 to 5 and 8 to 13 are effectivefor forming a zinc plating film excellent in qualities (appearance, easeof after treatment) even in practical embodiments using a barrel platingfor 60-minute current carrying time and the chemical conversiontreatment in combination.

INDUSTRIAL APPLICABILITY

The present invention can be used as a zincate zinc plating bath forforming a zinc plating film on automotive steel sheets, bolts, nuts, andthe like by using electroplating.

1-12. (canceled)
 13. A zincate zinc plating bath for depositing a metalcontaining at least zinc to form a zinc plating film by usingelectroplating, comprising a primary brightener having a function ofreducing the size of crystals of the metal thus deposited and a platingaccelerator containing a heterocyclic compound represented by thefollowing chemical formula (I):

(in the chemical formula (I), n stands for a natural number from 1 to 3,R¹ and R² may be same or different and each represents hydrogen,CH₂COOH, CH₂CH₂OH, or CH₂CH(OH)CH₂Cl) and/or a derivative of theheterocyclic compound.
 14. The zincate zinc plating bath according toclaim 13, wherein the heterocyclic compound and/or the derivative of theheterocyclic compound is contained in an amount of 0.01 g/L or greaterbut not greater than 1.0 g/L.
 15. The zincate zinc plating bathaccording to claim 13, wherein a water soluble cationic macromolecularcompound is contained as the primary brightener.
 16. The zincate zincplating bath according to claim 14, wherein a water soluble cationicmacromolecular compound is contained as the primary brightener.
 17. Thezincate zinc plating bath according to claim 15, wherein a compoundrepresented by the following chemical formula (II):

(in the chemical formula (II), n stands for a natural number of 1 orgreater, R¹, R², R³, and R⁴ may be the same or different and eachrepresents hydrogen, methyl, ethyl, isopropyl,2-hydroxylethyl-CH₂CH₂(OCH₂CH₂)_(x)OH (X stands for a natural numberfrom 0 to 6), or 2-hydroxyethyl-CH₂CH₂CH₂(OCCH₂CH₂)_(x)OH (X stands fora natural number from 0 to 6), R⁵ represents (CH₂)₂—O—(CH₂)₂,(CH₂)₂—O—(CH₂)—O—(CH₂)₂, or CH₂—CHOH—CH₂—O—CH₂—CHOH—CH₂, and Yrepresents S or O) is contained as the water soluble cationicmacromolecular compound.
 18. The zincate zinc plating bath according toclaim 16, wherein a compound represented by the following chemicalformula (II):

(in the chemical formula (II), n stands for a natural number of 1 orgreater, R¹, R², R³, and R⁴ may be the same or different and eachrepresents hydrogen, methyl, ethyl, isopropyl,2-hydroxylethyl-CH₂CH₂(OCH₂CH₂)_(x)OH (X stands for a natural numberfrom 0 to 6), or 2-hydroxyethyl-CH₂CH₂(OCCH₂CH₂)_(x)OH (X stands for anatural number from 0 to 6), R⁵ represents (CH₂)₂—O—(CH₂)₂,(CH₂)₂—O—(CH₂)—O—(CH₂)₂, or CH₂—CHOH—CH₂—O—CH₂—CHOH—CH₂, and Yrepresents S or O) is contained as the water soluble cationicmacromolecular compound.
 19. The zincate zinc plating bath according toclaim 13, further comprising a secondary brightener having a function ofplanarizing the zinc plating film.
 20. The zincate zinc plating bathaccording to claim 14, further comprising a secondary brightener havinga function of planarizing the zinc plating film.
 21. The zincate zincplating bath according to claim 15, further comprising a secondarybrightener having a function of planarizing the zinc plating film. 22.The zincate zinc plating bath according to claim 16, further comprisinga secondary brightener having a function of planarizing the zinc platingfilm.
 23. The zincate zinc plating bath according to claim 17, furthercomprising a secondary brightener having a function of planarizing thezinc plating film.
 24. The zincate zinc plating bath according to claim18, further comprising a secondary brightener having a function ofplanarizing the zinc plating film.
 25. The zincate zinc plating bathaccording to claim 19, wherein at least one compound selected from thegroup consisting of organic aldehydes and heterocyclic compounds iscontained as the secondary brightener.
 26. The zincate zinc plating bathaccording to claim 25, wherein at least either one of an aromaticaldehyde or a pyridinium compound is contained as the secondarybrightener.
 27. The zincate zinc plating bath according to claim 13,comprising zinc ions in an amount of 5 g/L or greater but not greaterthan 20 g/L.
 28. The zincate zinc plating bath according to claim 13,comprising sodium hydroxide in an amount of 80 g/L or greater but notgreater than 250 g/L.
 29. The zincate zinc plating bath according toclaim 13, comprising at least either one of nickel ions or iron ions.30. The zincate zinc plating bath according to claim 29, wherein thenickel ions are contained in an amount of 100 mg/L or greater but notgreater than 4000 mg/L.
 31. The zincate zinc plating bath according toclaim 29, wherein the iron ions are contained in an amount of 10 mg/L orgreater but not greater than 150 mg/L.